Irrespective of the internal combustion engine type (for example piston engine, rotary engine or free-piston engine), noises are generated due to the cycle of strokes (notably the induction and compression of an air fuel mix, power, and exhaust of the combusted air fuel mix). Part of the noise propagates through the combustion engine in the form of structure-borne noise and is then emitted from the combustion engine's outside in the form of airborne noise. Another part of the noise travels, together with the combusted air fuel mix, in the form of airborne noise through an exhaust system that is in fluid communication with the internal combustion engine. The noise traveling through the exhaust system in the form of airborne noise is called exhaust noise.
Other noises generated by vehicles driven by an internal combustion engine are the tire rolling noise on the roadway surface and the aerodynamic noise due to air displacement while the vehicle is moving.
These noises are often regarded as being harmful. Accordingly, there are statutory provisions for noise control to be observed by manufacturers of vehicles driven by internal combustion engines. The statutory provisions usually specify a maximum allowable sound pressure for a vehicle in operation. In addition, manufacturers try to give a distinctive noise emission to their vehicles driven by internal combustion engines, fitting the image of the respective manufacturer and favored by their customers. With state of the art small-displacement engines, it is no longer possible to achieve such a distinctive noise emission in a natural way.
Noise propagating through the internal combustion engine in the form of structure-borne noise can be deadened well and is therefore generally not an issue with regard to noise control.
Exhaust noise that propagates through an exhaust system of an internal combustion engine in the form of airborne sound, together with the combusted air fuel mix, is mitigated by mufflers located upstream of the discharge opening and, when present, downstream of catalytic converters. Respective mufflers may operate, for instance, according to at least one of the absorption and reflection principle. The drawback of both modes of operation is that they require a comparatively large volume, and build up a relatively high resistance to the combusted air fuel mix, thereby reducing the overall efficiency of the vehicle and increasing its fuel consumption.
For quite some time, so-called anti-noise systems have been developed as an alternative or complement to mufflers which add electro acoustically generated anti-noise to airborne noise generated in the internal combustion engine and traveling through the exhaust system. Respective anti-noise systems usually use a so-called Filtered-x Least Mean Squares (FxLMS) algorithm that tries to reduce the noise propagating through the exhaust system to zero (in the case of noise cancellation) by emitting sound from at least one sound generator (e.g. a voice coil loudspeaker or a different acoustic actor) that is in fluid communication with the exhaust system or to a predetermined threshold (in the case of active noise manipulation). To achieve complete destructive interference between the sound waves of the airborne noise traveling through the exhaust system and the anti-noise generated with the sound generator, the sound waves from the sound generator have to have the same amplitude and frequency as the sound waves propagating through the exhaust system but are shifted by 180 degrees in phase. When the sound waves propagating through the exhaust system have the same frequency as the sound waves of the anti-noise generated at the sound generator with their phases shifted by 180 degrees with respect to each other, but do not correspond in amplitudes, the sound waves of the airborne noise propagating through the exhaust system is only mitigated. The FxLMS algorithm calculates the anti-noise for each frequency band of the airborne noise propagating through the exhaust system separately by identifying an appropriate frequency and phasing for two sinusoidal oscillations that are shifted by 90 degrees with respect to each other and calculating the required amplitudes for these sinusoidal oscillations. Anti-noise systems aim at the noise cancellation or noise manipulation being audible and measurable at least outside of, but if need be also inside, the exhaust system. Establishing a control signal for generating a desired anti-noise with a sound generator is known to a person skilled in the art from documents U.S. Pat. No. 4,177,874, U.S. Pat. No. 5,229,556, U.S. Pat. No. 5,233,137, U.S. Pat. No. 5,343,533, U.S. Pat. No. 5,336,856, U.S. Pat. No. 5,432,857, U.S. Pat. No. 5,600,106, U.S. Pat. No. 5,619,020, EP 0 373 188, EP 0 674 097, EP 0 755 045, EP 0 916 817, EP 1 055 804, EP 1 627 996, DE 197 51 596, DE 10 2006 042 224, DE 10 2008 018 085 and DE 10 2009 031 848. A description of further details is therefore omitted. In this respect it is noted that the term “anti-noise” is used in this document to discriminate between the engineered sound from the sound generator and exhaust noise or other noises originating from the internal combustion engine. By itself, the anti-noise is nothing else than ordinary noise (usually airborne noise).
Creating noise from anti-noise systems may be implemented by coupling the sound generator acoustically to the exhaust system. As an alternative it is also known to mount the sound generator separately from the exhaust system, e.g. at the underbody of a vehicle rear, in order to emit the anti-noise from there. Irrespective of the sound generator being mounted in fluid communication with the exhaust system or separate from the exhaust system at a vehicle's underbody, placing the sound generator on the underbody of a vehicle causes several problems: firstly, the space available is usually very limited requiring a very compact design of the muffler, secondly, the sound generator has to be protected from environmental influences, and in particular from water and contamination.
As an example for respective sound generators, a sound generator for generating anti-noise in order to manipulate sound waves propagating through an exhaust system of a vehicle driven by an internal combustion engine is described below with respect to FIGS. 1A and 1B.
The sound generator 3 illustrated in the perspective view of FIG. 1A comprises an inherently stable two-part casing formed by an upper shell 32 and a lower shell 33 put together in an airtight manner. The casing houses an electrodynamic loudspeaker 2 and is connected to an exhaust system via a Y-pipe 1. At the base of the “Y”, the Y-pipe has a port 5 for discharging exhaust gas traveling through the exhaust system 4 and noise generated by the loud speaker 2. By having the connection implemented with the Y-pipe, the thermal stress of the loudspeaker 2 disposed within the sound generator 3 due to the exhaust gas traveling through the exhaust system 4 is kept low. This is necessary, because conventional loudspeakers are configured to be operated in a range up to a maximum of 200° C. only, while the temperature of the exhaust gases traveling through the exhaust system 4 may be between 400° C. and 700° C. A pressure compensation valve 36 is disposed on the upper shell 32 of the casing. To protect the pressure compensation valve 36 positioned on the surface of the upper shell 32 from being damaged, the upper shell 32 also supports a cast metal ring 37 surrounding the pressure compensation valve 36. The ring 37 has a slot at its bottom for allowing liquid to drain off from the region surrounded by the ring 37. Finally, the upper shell 32 holds a cable bushing 34 through which connecting wires are fed-through into the inside of the sound generator 3.
FIG. 1B shows a schematic cross section through the sound generator 3 of FIG. 1A. As can be seen, the loudspeaker 2 comprises a voice coil type loudspeaker 2, a permanent magnet 21, and a bell-mouthed membrane 22 which are together supported by a loudspeaker basket 23. Hereby, the membrane 22 is connected at its radial outside to the loudspeaker basket 23 by an elastic surround (not shown) and comprises at its radial inside a voice coil (not shown) that moves in bores formed in the permanent magnet 21. By applying an alternating current to the voice coil, a Lorentz force is exerted onto the membrane 22 by the voice coil resulting in an oscillation of the membrane 22. Wires 35 supply the control signals required for operating the voice coil through the cable bushing 34 disposed on the upper shell 32 of the casing. At its radial outside, the loudspeaker basket 23 is supported by a bell mouth 42 connected to the Y-pipe 1 via a connecting pipe 41. The bell-mouth 42 has to be used in the example shown, since the area of the loudspeaker's 2 membrane 22 is larger than the cross-sectional area of the exhaust system 4 in the sound coupling region. The large area of the membrane 22 is necessary to achieve the required sound energy flux. The bell-mouthed membrane 22 defines an axis of symmetry forming an angle of 33° with the bottom of the bell mouth 42. The membrane 22, the surround, a fringe of the loudspeaker basket 23, and the bell 42 divide the volume enclosed by the casing into a rear volume 38 that is not in fluid communication with the Y-pipe 1, and a front volume 39 that is in fluid communication with the Y-pipe 1. The rear volume 37 is thus basically sealed and acts as an air cushion onto the membrane 22 of the loudspeaker 2. The front volume 39 corresponds basically to the volume enclosed by the bell 42 and is not sealed. Depending on the (air) pressure in the rear volume 37 being higher or lower than the (air) pressure in the front volume 39, the rear volume 37 dampens the membrane 22 to a greater or lesser extent and may also cause a deflection of the membrane 22 to only one side from its zero position. Operating the loudspeaker 2 with a respective one-sided displacement of the membrane 22 from its zero position results in a considerable reduction of its life expectancy. The pressure compensation valve 36 ensures that a pressure inside the casing is approximately the same as a pressure outside of the casing. By providing the pressure compensation valve 36, the pressure inside the rear volume 38 is continuously adapted to the pressure present outside the casing of the sound generator 3. This is supposed to prevent a one-sided displacement of the membrane 22 from its zero position.
A drawback of the above configuration is that the sound generator's pressure compensation valve frequently functions unreliably. One reason being that the pressure compensation valve is easily damaged by impacts from the outside; the other that dust and water may easily clog the pressure compensation valve making any pressure compensation impossible. Since pressure compensation valves are often designed for air to pass through but not for water to pass through, pressure compensation is often not possible, particularly when the pressure compensation valve of the sound generator is located below the surface of a water body. Consequently, it is often necessary to use a loudspeaker of increased robustness inside the sound generator. This increases cost and may, due to the increased rigidity of the membrane involved therewith, and reduces the acoustic performance of the loudspeaker at low frequencies.
In order to solve this problem, DE 10 2013 208 186 A1 suggests to couple the pressure compensation valve to the sound generator via a long pressure compensation line, allowing the pressure compensation valve to be placed at any (and thus well protected) position on the vehicle. This, however, increases the effort for mounting the sound generator considerably.
A further problem with the configuration described above is that, when the exhaust system is submerged into water, an increased pressure is applied from outside to the membrane. This results in the membrane no longer oscillating around its rest position but rather on a plane spaced from this rest position, and thus having an offset. An oscillation of the offset membrane further results in the rear volume being pumped out through the pressure compensation valve.
The above problems are also present, when the sound generator is not in fluid communication with the exhaust system different to the sound generator shown in FIGS. 1A and 1B. Also in this case the membrane and a casing of the sound generator enclose a rear volume so that a pressure compensation valve is also required here. For a sound generator that is not in fluid communication with the exhaust system there is also an increased risk of a membrane offset due to an increased outside pressure.